This invention relates generally to screens for use in conjunction with optical image projectors, and more particularly to an inflatable screen adapted to function, when erected, as a rear projection or a front projection screen.
In order for a sheet to function effectively as a viewing screen for images projected onto the front surface thereof, the surface must be of a reflective character and have diffusion properties. Thus the conventional rear projection screen is formed of opaque reflective material having diffusion properties which causes light impinging thereon to be scattered in all directions. A perfect or Lambertian diffuser is one that has the same apparent brightness from any angle.
Matte white paper is a good reflecting diffuser, for it reflects 70 to 80% of the incident visible light. But in front projection screens for use with slide or movie projectors, higher efficiencies are required, particularly with non-professional or home equipment having relatively low wattage lamps. For this purpose, opaque screen surfaces having magnesium oxide and magnesium carbonate particles embedded therein are frequently used; for their efficiencies are high, in the order of 97 or 98%.
With rear projection screens, transmissive diffusing material must be used to produce even illumination. Thus transparent glass or acrylic materials containing a suspension of minute colloidal particles act to diffuse light passing therethrough because of multiple scattering from these particles. Ground glass is also effective as a rear projection screen, for the multitude of very small facets produced by etching or fine grinding refract the incident light more or less randomly.
It is known to use in conjunction with home movie or slide projectors collapsible front projection screens which are stored in rolled-up condition in a cylindrical housing. Such screens can be unwound and erected, the erected screen being supported on a tripod stand. Structures of this type are relatively expensive and can function only as front projection screens.
Moreover, while highly compact slide projectors are commercially available--and these can be conveniently carried--this is not true of collapsible screens of the type heretofore known, for such screens are relatively cumbersome. It is for this reason that slides are often projected against a blank wall, even though this surface does not function efficiently as a viewing screen.
A need also exists for a screen that can serve either as a front or rear projection screen. Thus when slide projectors are used in a classroom, in order for projected slides to be viewable by students seated in the room, a front projection screen must be erected in front of the class. This makes it necessary to place the slide projector toward the rear of the classroom. If an instructor wishes, while showing slides, to comment thereon from a position in front of the class and yet able to control the slide presentation, a remote control link must be set up between the instructor and the slide projector.
A more convenient arrangement would be to have the slide projector placed in front of the class next to the instructor, with the slide images cast on a rear projector screen facing the seated students. But since there are no commercially-available collapsible or portable rear projection screens, it has not heretofore been possible to set up this more convenient arrangement.